The present invention relates to ultrasound transducers and, in particular, it concerns cylindrical ultrasound receivers and transceivers formed from piezoelectric films, and their applications in digitizer systems.
It is known to employ cylindrical ultrasound transducers for transmitting ultrasound signals in digitizer systems. The cylindrical form provides all-around signal transmission and simplifies the geometry of time-of-flight calculations by providing an effect similar to a point (or more accurately, line) source. These advantages are detailed in U.S. Pat. No. 4,758,691 to De Bruyne. A further advantage of cylindrical ultrasound transducers is that they can be centered on an element of which the position is to be measured. This is used in a drawing implement digitizer system described in PCT publication WO98/40838.
Structurally, a number of different types of cylindrical transducer have been proposed. The De Bruyne patent proposes a xe2x80x9cSell transducerxe2x80x9d which is a capacitive device formed from a complicated arrangement of cylindrical layers intended to produce a cylindrical air gap of about 20 xcexcm. Such a structure is costly to manufacture, and is likely to be unreliable.
A second type of transducer that has been proposed in the field of medical applications is based on piezoelectric elements. An example of a medical transducer of this type may be found in U.S. Pat. No. 4,706,681 to Breyer et al., which discloses an ultrasonic marker. Here, a cylindrical piezoelectric collar is sandwiched between two electrodes. Application of an alternating potential across the electrodes causes vibration of the collar, and hence emits a radially propagating ultrasonic signal.
In principle, any ultrasonic transducer is capable of being operated both as a transmitter and a receiver. In practice, however, many considerations result in many transmitter structures being ineffective as receivers. This is particularly true of cylindrical elements in which almost the entire cylinder contributes to wide angle transmission by actuation with a relatively high power while only a small portion of the cylinder is correctly orientated for receiving an incoming signal from a given direction. Furthermore, the inherent capacitance of the large inactive region of the transducer may absorb a large proportion of the amplitude of a received signal, rendering the transducer insensitive as a receiver.
In the field of transducers in general, much work has been invested in development of devices based on piezoelectric films, such as PVDF. Conductive electrodes are formed on opposite faces of the film, typically by selectively printing conductive ink on regions of the surfaces. These films are cheap to produce, and withstand a wide range of operating conditions including exposure to moisture.
Although a cylindrical ultrasound transducer is relatively simple to implement using piezoelectric film, implementation of a receiver poses additional problems beyond the general complications of cylindrical receivers discussed above. Specifically, referring to FIGS. 1, 2 there is shown a schematic plan view of a freely suspended cylinder 10 formed from piezoelectric film. FIG. 1 shows its relaxed state, while FIG. 2 shows the response of cylinder 10 to an incoming ultrasound signal wave front 15. Since the piezoelectric film is flexible, the oscillations of signal 15 generate waves (exaggerated for clarity) traveling around cylinder 10. The direction and extent of flexing of the piezoelectric film varies along the waveform created around the cylinder, resulting in reversal of the sense of an electrical potential generated between the electrodes. As a result, much of the potential generated by the piezoelectric film may be dissipated in local eddy currents within the electrodes, greatly reducing the overall signal voltage as measured between the electrodes.
A further problem of implementing a cylindrical ultrasound transducer using piezoelectric film is the tendency for the electrode to act as an antenna, picking up unwanted electromagnetic radiation which may result in very low signal to noise ratios.
A further problem of implementing a cylindrical ultrasound transducer using piezoelectric film is to provide mechanical protection for the transducer while minimizing disruption of the ultrasound waves.
A further problem of implementing a cylindrical ultrasound transducer using piezoelectric film is the damage caused through welding the piezoelectric film to form a cylinder.
There is therefore a need for a cylindrical ultrasound receiver structure employing piezoelectric film.
The present invention is a cylindrical ultrasound receiver structure employing piezoelectric film.
According to the teachings of the present invention there is provided an ultrasound transducer comprising: (a) a piezoelectric film having a first end and a second end; (b) a plurality of electrodes disposed on the piezoelectric film; (c) at least one securing member; and (d) a support structure, which is substantially cylindrical, wherein the first end and the second end are secured to the support structure by the at least one securing member.
According to a further feature of the present invention, there is also provided an electrical contact disposed on the support structure.
According to a further feature of the present invention, the support structure further includes a protrusion and wherein the first end and the second end are secured to the protrusion by the at least one securing member.
According to a further feature of the present invention: (a) the support structure has a central axis; (b) the protrusion is formed as an elongated projecting ridge having a direction of elongation; and (c) the direction of elongation being substantially parallel to the central axis.
According to a further feature of the present invention, there is also provided an electrical contact disposed on the protrusion.
According to a further feature of the present invention, the at least one securing member is a clip.
According to a further feature of the present invention, there is also provided an electrical contact wherein the electrical contact is disposed on the at least one securing member.
According to a further feature of the present invention, the piezoelectric film has a first surface and a second surface and wherein the electrodes include: (a) a first electrode disposed on the first surface; (b) a second electrode disposed on the second surface wherein at least a part of the second electrode is in an opposing relationship with at least a part of the first electrode; (c) a first electrical connecting strip disposed on the first surface wherein the first electrical connecting strip is connected to the first electrode; and (d) a second electrical connecting strip disposed on the second surface in a substantially non-opposing relationship with the first electrical connecting strip wherein the second electrical connecting strip is connected to the second electrode.
According to a further feature of the present invention, the piezoelectric film has a first surface and a second surface and wherein the electrodes include: (a) a first electrode and a second electrode disposed on the first surface, wherein the first electrode is disposed in a pattern that is non-contiguous with the second electrode; (b) a third electrode and a fourth electrode disposed on the second surface, wherein: (i) at least a part of the third electrode is in an opposing relationship with at least a part of the first electrode; (ii) at least a part of the fourth electrode is in an opposing relationship with at least a part of the second electrode; and (iii) the third electrode is disposed in a pattern that is non-contiguous with the fourth electrode; and (c) an electrical joining strip extending from the first electrode to the fourth electrode, wherein the electrical joining strip includes a first portion of the electrical joining strip on the first surface and a second portion of the electrical joining strip on the second surface, and wherein the first portion and the second portion are electrically connected.
According to a further feature of the present invention, the first portion and the second portion are electrically connected via a hole in the piezoelectric film.
According to a further feature of the present invention, there is also provided a helical metal spring, wherein the helical metal spring is disposed around the piezoelectric film.
According to additional teachings of the present invention there is also provided an ultrasound receiver comprising: (a) a piezoelectric film having a first surface and a second surface; (b) a first electrode disposed on the first surface; (c) a second electrode disposed on the second surface wherein at least a part of the second electrode is in an opposing relationship with at least a part of the first electrode; (d) a first electrical connecting strip disposed on the first surface wherein the first electrical connecting strip is connected to the first electrode; and (e) a second electrical connecting strip disposed on the second surface in a substantially non-opposing relationship with the first electrical connecting strip wherein the second electrical connecting strip is connected to the second electrode.
According to a further feature of the present invention, the first electrical connecting strip is in a substantially non-opposing relationship with the second electrode; and the second electrical connecting strip is in a substantially non-opposing relationship with the first electrode.
According to a further feature of the present invention, there is also provided a substantially cylindrical element, which is hollow, formed primarily from the piezoelectric film, the substantially cylindrical element having a central axis and a height measured parallel to the central axis; and a support structure for supporting the substantially cylindrical element, the support structure being configured to support the substantially cylindrical element in such a manner as to allow propagation of vibration waves circumferentially around a major part of the substantially cylindrical element; wherein the first electrode is formed as a strip extending in an extensional direction substantially parallel to the central axis along at least a part of the height, the strip subtending at the central axis an angle of not more than 90xc2x0.
According to a further feature of the present invention, the substantially cylindrical element has an inner surface wherein the first surface forms the inner surface; and the second electrode is grounded.
According to additional teachings of the present invention there is also provided a multi-electrode ultrasound receiver comprising: (a) a piezoelectric film having a first surface and a second surface; (b) a first electrode and a second electrode disposed on the first surface, wherein the first electrode is disposed in a pattern that is non-contiguous with the second electrode; (c) a third electrode and a fourth electrode disposed on the second surface, wherein: (i) at least a part of the third electrode is in an opposing relationship with at least a part of the first electrode; (ii) at least a part of the fourth electrode is in an opposing relationship with at least a part of the second electrode; and (iii) the third electrode is disposed in a pattern that is non-contiguous with the fourth electrode; and (d) an electrical joining strip extending from the first electrode to the fourth electrode wherein the electrical joining strip includes a first portion of the electrical joining strip on the first surface and a second portion of the electrical joining strip on the second surface and the first portion and the second portion being electrically connected.
According to a further feature of the present invention, there is also provided a substantially cylindrical element, which is hollow, formed primarily from the piezoelectric film, the substantially cylindrical element having a central axis and a height measured parallel to the central axis and wherein the first electrode and the second electrode in combination subtend at the central axis an angle of not more than 90xc2x0; and a support structure for supporting the substantially cylindrical element, the support structure being configured to support the substantially cylindrical element in such a manner as to allow propagation of vibration waves circumferentially around a major part of the substantially cylindrical element.
According to a further feature of the present invention, the substantially cylindrical element has an inner surface wherein the first surface forms the inner surface; and the third electrode is grounded.
According to a further feature of the present invention, the first portion and the second portion are electrically connected via a hole in the piezoelectric film.
According to a further feature of the present invention, there is also provided a first electrical connecting strip disposed on the first surface, wherein the first electrical connecting strip is connected to the second electrode; and a second electrical connecting strip disposed on the second surface, wherein the second electrical connecting strip is connected to the third electrode and the second electrical connecting strip is in a substantially non-opposing relationship with the first electrical connecting strip.
According to additional teachings of the present invention there is also provided a method for providing shielding for an ultrasound transducer used for a predetermined frequency of ultrasound waves while minimizing disruption to the ultrasound waves, comprising the steps of spacing windings of a helical metal spring at a spatial period of less than about half of a wavelength of the ultrasound waves associated with the ultrasound transducer; and positioning the helical metal spring surrounding the ultrasound transducer.
According to a further feature of the present invention, the step of spacing is performed by spacing the windings at a spatial period of less than about quarter of the wavelength.
According to additional teachings of the present invention there is also provided a digitizer system comprising: (a) an ultrasound transducer associated with a moveable element; (b) two ultrasound transducers; (c) a base unit; wherein the two ultrasound transducers are maintained in fixed geometrical relation by attachment to the base unit; and (d) an acoustic wave-guide; wherein the acoustic wave-guide includes a hollow elongated member and the acoustic wave-guide is disposed between the two ultrasound transducers.
According to a further feature of the present invention, the acoustic wave-guide is substantially straight.
According to a further feature of the present invention, the acoustic wave-guide is curved.
According to additional teachings of the present invention there is also provided a method for operating a system for determining a position of a point on a moveable element, the system including: a moveable group of ultrasound transducers including a first ultrasound transducer and a second ultrasound transducer each mounted on the moveable element where the first ultrasound transducer, the second ultrasound transducer and the point on the moveable element are sequentially spaced along a common axis; and a fixed group of ultrasound transducers including a third ultrasound transducer and a fourth ultrasound transducer spaced apart by a predefined distance, the method for operating comprising the steps of: (a) transmitting a plurality of measurement signals between the first ultrasound transducer and the fixed group and between the second ultrasound transducer and the fixed group; (b) deriving distances between the first ultrasound transducer and each of the third ultrasound transducer and the fourth ultrasound transducer and between the second ultrasound transducer and each of the third ultrasound transducer and the fourth ultrasound transducer from time-of-flight measurements for the measurement signals; and (c) deriving from the distances a position of the point.
According to a further feature of the present invention, the first ultrasound transducer and the second ultrasound transducer are both cylindrical ultrasound transducers.